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<a name="hash.custom"></a><a class="link" href="custom.html" title="Extending boost::hash for a custom data type">Extending boost::hash for a custom data type</a>
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<p>
      <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> is implemented by calling
      the function <code class="computeroutput"><a class="link" href="reference.html#boost.hash_value_idp39522320">hash_value</a></code>.
      The namespace isn't specified so that it can detect overloads via argument
      dependant lookup. So if there is a free function <code class="computeroutput"><span class="identifier">hash_value</span></code>
      in the same namespace as a custom type, it will get called.
    </p>
<p>
      If you have a structure <code class="computeroutput"><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span></code>, where
      each <code class="computeroutput"><span class="identifier">book</span></code> is uniquely defined
      by it's member <code class="computeroutput"><span class="identifier">id</span></code>:
    </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">library</span>
<span class="special">{</span>
    <span class="keyword">struct</span> <span class="identifier">book</span>
    <span class="special">{</span>
        <span class="keyword">int</span> <span class="identifier">id</span><span class="special">;</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">author</span><span class="special">;</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">title</span><span class="special">;</span>

        <span class="comment">// ....</span>
    <span class="special">};</span>

    <span class="keyword">bool</span> <span class="keyword">operator</span><span class="special">==(</span><span class="identifier">book</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">a</span><span class="special">,</span> <span class="identifier">book</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">b</span><span class="special">)</span>
    <span class="special">{</span>
        <span class="keyword">return</span> <span class="identifier">a</span><span class="special">.</span><span class="identifier">id</span> <span class="special">==</span> <span class="identifier">b</span><span class="special">.</span><span class="identifier">id</span><span class="special">;</span>
    <span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
      Then all you would need to do is write the function <code class="computeroutput"><span class="identifier">library</span><span class="special">::</span><span class="identifier">hash_value</span></code>:
    </p>
<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">library</span>
<span class="special">{</span>
    <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">hash_value</span><span class="special">(</span><span class="identifier">book</span> <span class="keyword">const</span><span class="special">&amp;</span> <span class="identifier">b</span><span class="special">)</span>
    <span class="special">{</span>
        <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span> <span class="identifier">hasher</span><span class="special">;</span>
        <span class="keyword">return</span> <span class="identifier">hasher</span><span class="special">(</span><span class="identifier">b</span><span class="special">.</span><span class="identifier">id</span><span class="special">);</span>
    <span class="special">}</span>
<span class="special">}</span>
</pre>
<p>
      And you can now use <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code> with
      book:
    </p>
<pre class="programlisting"><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span> <span class="identifier">knife</span><span class="special">(</span><span class="number">3458</span><span class="special">,</span> <span class="string">"Zane Grey"</span><span class="special">,</span> <span class="string">"The Hash Knife Outfit"</span><span class="special">);</span>
<span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span> <span class="identifier">dandelion</span><span class="special">(</span><span class="number">1354</span><span class="special">,</span> <span class="string">"Paul J. Shanley"</span><span class="special">,</span>
    <span class="string">"Hash &amp; Dandelion Greens"</span><span class="special">);</span>

<code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special">&lt;</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">&gt;</span> <span class="identifier">book_hasher</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">knife_hash_value</span> <span class="special">=</span> <span class="identifier">book_hasher</span><span class="special">(</span><span class="identifier">knife</span><span class="special">);</span>

<span class="comment">// If std::unordered_set is available:</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">unordered_set</span><span class="special">&lt;</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">,</span> <code class="computeroutput"><a class="link" href="../boost/hash.html" title="Struct template hash">boost::hash</a></code><span class="special">&lt;</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">&gt;</span> <span class="special">&gt;</span> <span class="identifier">books</span><span class="special">;</span>
<span class="identifier">books</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">knife</span><span class="special">);</span>
<span class="identifier">books</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">(</span><span class="number">2443</span><span class="special">,</span> <span class="string">"Lindgren, Torgny"</span><span class="special">,</span> <span class="string">"Hash"</span><span class="special">));</span>
<span class="identifier">books</span><span class="special">.</span><span class="identifier">insert</span><span class="special">(</span><span class="identifier">library</span><span class="special">::</span><span class="identifier">book</span><span class="special">(</span><span class="number">1953</span><span class="special">,</span> <span class="string">"Snyder, Bernadette M."</span><span class="special">,</span>
    <span class="string">"Heavenly Hash: A Tasty Mix of a Mother's Meditations"</span><span class="special">));</span>

<span class="identifier">assert</span><span class="special">(</span><span class="identifier">books</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">knife</span><span class="special">)</span> <span class="special">!=</span> <span class="identifier">books</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
<span class="identifier">assert</span><span class="special">(</span><span class="identifier">books</span><span class="special">.</span><span class="identifier">find</span><span class="special">(</span><span class="identifier">dandelion</span><span class="special">)</span> <span class="special">==</span> <span class="identifier">books</span><span class="special">.</span><span class="identifier">end</span><span class="special">());</span>
</pre>
<p>
      The full example can be found in: <a href="../../../libs/functional/hash/examples/books.hpp" target="_top">/libs/functional/hash/examples/books.hpp</a>
      and <a href="../../../libs/functional/hash/examples/books.cpp" target="_top">/libs/functional/hash/examples/books.cpp</a>.
    </p>
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<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../doc/src/images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top"><p>
        When writing a hash function, first look at how the equality function works.
        Objects that are equal must generate the same hash value. When objects are
        not equal they should generate different hash values. In this object equality
        was based just on the id so the hash function only hashes the id. If it was
        based on the object's name and author then the hash function should take
        them into account (how to do this is discussed in the next section).
      </p></td></tr>
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<td align="right"><div class="copyright-footer">Copyright &#169; 2005-2008 Daniel
      James<p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
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